The Mattole Basin encompasses approximately 296 square miles of Northern California’s Coast Range (Figure 7). Although nearly three percent of the Mattole’s headwaters are in Mendocino County; the vast majority of the basin is within Humboldt County. The mainstem Mattole River is approximately 62 miles long, and receives water from over 74 tributary streams. There are approximately 545 perennial stream miles in the basin. The basin drains into the Pacific Ocean just south of Cape Mendocino. During most summers, a sand-spit encroaches all the way across the river mouth to form a bay mouth barrier, which creates a lagoon behind it. Generally, the barrier remains until runoff from fall rains breeches it. However, in some years, large swells at times of high tide overtop the barrier and a new outlet channel is carved through the barrier. This overtopping has occurred up to six times during a year before the lagoon finally remained closed.

The Mattole Basin is mostly steep mountainous topography. The basin’s higher elevation slopes commonly exceed 15 percent gradient. Broad, alluvial streamside flats are present in the lower valleys. The lower stream channels are dominated by large gravel bars typically composed of cobble, gravel, and fine sediments (Elements of Recovery, 1989). Headwater elevations range from 1,350 feet at Four Corners at the mainstem headwaters, to 4,088 feet at Kings Peak, which is located less than three miles from the ocean and is the tallest mountain in the coastal range.